During the past years, the interest in radio access technologies for providing services for voice, video and data has increased. There are various telecom technologies used in cellular communications. Widespread radio access technologies for mobile communication are digital cellular. Increased interest is shown in 3G (third generation) systems. Within the 3rd generation partnership program (3GPP), the standard for 3G long term evolution (LTE) system has been developed and can be found in 3GPP TR 36.300, Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); Overall description; Stage 2, V8.2.0., the entire content of which is incorporated herein by reference. It is envisioned that a future LTE-Advanced system will be based on LTE and that it will fulfill or exceed the IMT-Advanced requirements.
The LTE concept is based on orthogonal frequency division multiplexing (OFDM) for the downlink (i.e., the communication link from a base station to a terminal) and discrete Fourier transform spread OFDM (DFTS-OFDM) for the uplink (i.e., the communication link from the terminal to the base station). The uplink transmissions from different terminals (for example mobile stations) may be performed on a shared physical channel. Uplink transmissions are kept orthogonal by a scheduler located in the base station. The scheduler provides the transmitting mobile stations with uplink transmission grants that do not overlap in the shared time and frequency resource. Since the uplink transmissions are under tight control of the scheduling base station, there is an inherent delay of (at least) one transmission time interval (TTI) in the uplink. More specifically, the base station signals first the grants for uplink transmissions to the terminals using the downlink control channel or other channels. Then, the terminals need some time to decode the downlink control information received from the base stations before the terminals may transmit data on the granted resource, back to the base station.
Thus, there is a delay in all terminals when transmitting information to the base station due to the architecture of the uplink transmissions. The presence of multiple terminals communicating with the same base station and the implications of this scenario are discussed later, after other concepts of the LTE are discussed.
Within 3GPP, the work on an evolved high speed packet access (HSPA) concept is ongoing, as described for example in R1-080909, Requirements for E-DCH TDM support, Ericsson, the entire content of which is incorporated by reference. The uplink concept in HSPA is also based on a scheduler present in the base station. The scheduler issues grants for uplink transmission and these grants are sent to the terminals. However, the control of the uplink channel usage in HSPA is not as tight as in LTE. The grants issued in HSPA represent a maximum rate that a terminal is allowed to transmit with. If a terminal does not need to transmit at the granted rate, the terminal may transmit at a lower rate (including rate zero) then what the grant stipulates.
Simultaneous transmissions in the HSPA uplink are not orthogonal and thus, the scheduler has to issue grants (i.e., maximum transmission rates) such that the total uplink interference level is kept under control. In order to support higher data rates, the developers of the HSPA concept are considering introducing time domain multiple access (TDMA) in the uplink. With TDMA, the uplink of an evolved HSPA system may become similar to the orthogonal uplink of LTE.
Both the LTE and evolved HSPA processes discussed above support the concept of multi user multiple input multiple output (MU-MIMO), which is described in R1-080774, UL MU-MIMO scheduling for high mobility, Nortel, the entire content of which is incorporated here by reference. As with ordinary MIMO, i.e., transmission and reception from multiple antennas disposed at least at one of the receiver or transmitter, the MU-MIMO arrangement is used to increase the transmitted data rate on the communication channel. Since each terminal is equipped with at least one transmit antenna, a multiple user uplink is multiple input by nature. By scheduling two or more uplink users on the same physical resource, while assuring that the base station is equipped with a large enough number of receiving antennas, possibilities for spatial separation of the transmitting users are created by a receiver making use of traditional MIMO processing.
However, the increased transmission data rate in MU-MIMO arrangements that use either LTE or HSPA comes with a price in terms of increased received complexity. In the following, a serving base station refers to that base station to which a given terminal is connected and a neighboring base station is a station from which the terminal may receive an interfering signal but this base station is not providing the communication data to the terminal.
Inter-cell interference is one aspect of the broad interference phenomenon that affects a communication system. The inter-cell interference is illustrated in FIG. 1 and is produced by signals A and B from at least two terminals 10 and 12 that are in two different cells 14 and 16. The signals A and B are received by a same base station 18, i.e., a serving base station 18 for terminal 10 and not for terminal 12. In this regard, in an LTE system, a cell maintains a list of neighbor cells which is relevant for handover candidates. For each neighbor, the cell may store cell identities (for example a non-unique physical identity used when reporting measurements and a unique identity), connectivity information (e.g., an IP address of the eNB associated with the neighbor cell, the connectivity between eNBs, (i.e., X2 or S1 interfaces in LTE), and additional cell-specific information (e.g., handover algorithm details, type of cell, etc).
One way to manage the inter-cell interference is to control the transmission power to reduce the interference. Other ways to manage the inter-cell interference is to use one of inter-cell interference randomization, cancellation or coordination. Each of these methods aim at minimizing the degradation of the signals caused by the inter-cell interference. For example, inter-cell interference randomization methods may make use of cell-specific scrambling codes and/or cell-specific interleaving sequences to whiten the interference.
Inter-cell interference cancellation is based on detecting inter-cell interference and subtracting it from a received signal. Inter-cell interference coordination is based on coordinating transmission between cells in a static, semistatic or dynamic way for reducing the interference. Such coordination may require restrictions on resource allocation, for example on time and frequency resources and transmit power on those resources. The above noted inter-cell interference methods are discussed in more details in “Intercell interference management in an OFDM-based downlink,” J. Heyman, Master thesis, Linköpings universitet, Department of Electrical Engineering, 2006, the entire content of which is incorporated herein by reference. However, because interference coordination methods aim at avoiding interference while interference cancellation methods aim at removing interference, these methods are different, even though they address the same problem. Typically, the use of interference cancellation methods provides higher system capacity than interference coordination methods.
While MU-MIMO is a promising technique that enables spatial processing of intra-cell interference, this technique does not address the problem of inter-cell interference. Further, the above discussed uplink inter-cell interference coordination techniques operate on a slow time scale compared to the change rate of the uplink grant assignments issued by the schedulers, which defeats the purpose of the inter-cell interference reduction techniques.
Accordingly, it would be desirable to provide devices, systems, and methods that address the above mentioned shortcomings, problems and drawbacks and also provide a solution that enhances the performance of uplink transmissions in a communication system.